 The estrus cycle of rats can be quickly identified by observing the vaginal smears under a microscope. The smears can be examined at either 10x or 40x magnification, depending on the type of cells present. At 10x magnification, the smears will show the presence of three different cell types, squamous epithelial cells, glandular epithelial cells, and leukocytes. At 40x magnification, the smears will show more detail and allow for the identification of individual cells. By comparing the number of each cell type present in the smear, the researcher can determine the stage of the estrus cycle. This article was authored by F. K. Marcons, F. J. B. N. C. H. I. N. A. P. Tano.